deck on expansive soil

huangyhg

deck on expansive soil
i am really confused on building residential decks.  what is the "proper" way to support a deck on expansive soil.  people usually just dig a hole frost depth with sona tube, poor concrete and then put deck column on top of it.
now, if i have a brand new constuction and the soil report is telling me to use drilled piers, what do i do?  do i really need 20ft drilled pier (per minimum depth from soil report), or can i just use the bearing pressure and not worry about the minimum dead load?
what if you have a home owner that wants to build a deck?  do i need to tell them to get  a soil report?  it seems that getting a deck with the proper route is getting more and more expensive (engineer+soil report+contractor).  am i missing something here?
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if you second guess your geotech and something happens to the deck, you are on the hook for any damage.  
if you have questions, and you have a soils reoport, ask the geotech and document his answer.  that is what he is there for.  it's cya.
mike mccann
mccann engineering
i am not second guessing anything.  i just want to know what steps you need to take to build a deck.  do you need soil report?  if not, what is the bearing pressure you use?  how about if it is on expansive soil?  etc.
ok.
if you are dealing with a new deck add-on to an existing structure, then i would examine the existing structure to see if it has any distress, noting the type of construction for the foundation if observable.  if there are any drawings, either from the owner or local bd, that would be heolpful too, but they are probably on the moon.  
that being said, i would have to rely on the performance of the existing structure, or others nearby, to make my decision.  you may even be able to obtain a soils report from an adjacent structure if you are lucky.
it appears to me that what you are looking to do is save the client money by taking on additional risk yourself. that's your call, but professionally, i would not do it.  i would get the soils report.  for  2 to 3 grand, the piece of mind is worth it to me.
mike mccann
mccann engineering
how bout asking the geotech to recommend how much expansive soil needs to be removed and replaced with non-expansive soil, skipping the drilled piers altogether?
if expansive soils (or other 'extreme conditions) are an issue, the use of helical piers may be a cost effective option. check with a/the geotech.